Continuously checked communication system



Jan. 27, 1948. yJ'. w. KNAPP ET AL. 2,435,0W

CONTINUOUSLY CECKED COMMUNICATION SYSTEM Filed Nov.. 27, 1944 5 sheets-sheet 1 y vuw@ ois@ n l rfi@ md l l 1 I I l I ||L Y nventors `lVVKrzcLpp and JCOBien mm M ' Their' Comeg Jan. 27, i948. J. w. KNAPP ETAL. 2,4350@ CONTINUOUSLY CHECKED COMMUNICATION SYSTEM Filed Nov. 27, 1944 3 Sheets-Sheet 2` Bnvcntors J. WKnapp. and JCO'Brien Their" Cttorneg I jan. 27, 1948. J, W, KNAPP ETAL 2,435,010

CONTINUOSLY CHECKED COMMUNICATION SYSTEM Filed Nov. 27, 1944 5 Sheets-Sheet 3 JW. Knapp and J. COBrien Their Gttorneg Patented Jan. 27, 1948 coNTrNnoUsLY CHECKED coMMUNIcA. 'rroN SYSTEM.

James W; Knapp, Brockport, and John C. OBrien, Rochester, N. Y., assgnors to General Railway Signal Company, Rochester, N'. Y..

Application November 27, 1944, Serial No.. 565,206

91 Claims. l

Thisy invention relates to radio telephone com'- muni'cation apparatus,l and more particularly pertains to the inter-relation of a transmitting apparatus and' its associated receiving apparatus located at a central station providing two-Way communication between such central station .and a distant station.A

In radio communication systems employed in connection with the governing of trafc, such as on railroads and the like, it is considered desirable to provide. means of checking the continue'd' operativeness of the` system by intermittently transmitting; a checking signalfrom the central station to a distant station at which a suitable indication is displayed'. in. the event that the checkingl signalv fails to be received. In such a system, it is desirable that. during regular transmission that this. checking signal be removed, and that the. cessation of the. reception of such checking signal shall b e immediately followed. by proper voice communication, so that. if the operator at the distant station fails. to receive voice communication upon the cessation oi'. the checking signal', he Will be immediately advised that the; communication systemhas failed.. Itis also desirable. in a, system employing the checking feature where the same carrier. frequency channel' is used for. both directions of, transmission, to cause. the. cessation of the. checking. Signal` transmission from the central station while the distant station, is.` transmitting. This is. be.- cause the transmission. of the. checking signal' in such a case interferes with the proper reception of the. message being transmitted from the .distant station.

In` view of. the above considerations,1 it, is pro.- posed in. accordance with the present invention to providev means at. the, central station responsive to the reception. of. a carrier from. the distant station to render the. .checking signal trans: mitti'ng means. inefiective and at the same, time to. prevent the central'. operator from interrupt.- i'ng communication from the. outlying. station.,

In radio communication ysystems of the above type which employ amplitude modulation, of the muting the amplifier whenever a carrier is not being received and this means being so arranged as to require thereception of a carrier substantially above the noise level of the location before the amplifier is rendered active.

Therefore, another object of the present invention is to provide means which is controlled in accordance. with the reception 'of a carrier signal of a substantial value above the surrounding radio noise level in such a way that such means may be employed both for interlocking the receiver of the central oiiice with its associated transmitter, and also for the squelching or routing control ofv the `amplifying system of such receiver.

A further object of the invention is to accomplish the, above purposes and objects with a minimum amount of apparatus and in a. manner that the means referred' to Shall be of a, rugged and reliable type,..such as an electromagnetic relay of substantial. construction.

Other objects, purposes and characteristic features of the present invention will be in part obvious vfrom the accompanying drawings, and in. part pointed yout as the description of the invention progresses.

In describing the invention in detail, reference will be made to the accompanying drawings in which like reference characters designate corresponding parts throughout the several views, and in Which- Fig. l illustrates. in a diagrammatic manner the general organization and inter-relation of the radio transmitter and receiver apparatus. employed at a. central station as organized in accordance with the present invention;

Fig. 2 illustrates. ina diagrammatic manner one form of the. present invention as` providing or theV control of a squelch relay employed in the organization of Fig. I; Fig... 3 illustrates. in a diagrammatic. manner another form oi the present` invention as providing, for the control ofY a` squelchv relay and employed in the organization of Fig. 1.

For the purpose of simplifying the illustration and facilitating in. the explanation, the various parts. and circuits constituting the embodiment of the invention have been shown diagrammatically and certain. conventional illustrations have .been employed.. the drawings having been made more with. the purpose of making it easy to understand the. principles and mode of operation, than with the idea. of illustrating the specic construction vand arrangement of parts .that would ybe employed in practice. Thus, the

various electron tubes and their elements as Well as the various relays and their contacts are illustrated in a conventional manner, and symbols are used to indicate connections to the terminals of sources of electric current instead of showing all of the Wiring connections to these terminals.

The symbols (-I) and are employed to indicate the positive and negative terminals respectively of suitable sources of direct current; and the circuits with which these symbols are used always have current flowing in the same direction. The symbol (B+) isused to indicate the positive terminal of a suitable direct current source for the so-called B battery supplv of the electron tubes, it being assumed that the opposite terminal of such a source is connected to ground indicated by the usual convention. Similarly, the symbol (C-) is used to indicate the negative terminal of a suitable direct current source for providing the desired negative bias on theelectron tubes, it being assumed that the other terminal of this source is also connected to ground. It is of course understood that these diierent B and C sources of potential may be provided by suitable batteries, or may be provided by the usual conventional power pack organization. Since it is desirable to employ an alternating current in connection with the present invention, .the terminals connected to such a source of energy are designated by the legend A. C. Supply. The supply of energy to the heater elements of the cathodes of the various electron tubes has not been shown, but is assumed to be conventionally arranged for continuous operation.

Referring to Fig. 1, a radio transmitting apparatus has been indicated by a dotted rectangle designated Radio transmitter"; and the associated receiving apparatus has been indicated by another dotted rectangle designated "Radio receiver. Associated with the radio transmitter and receiver is a suitable antenna A which is normally connected through back contact B of relay RII to the radio receiver, but is connected through front contact 6 to the radio transmitter when the relay RII is energized or picked up. Similarly, a source of positive plate potential (B+) is supplied tothe radio receiver through back contact 'I of relay RII but when relay RZI is picked up, this (B+) is connected to the radio transmitter.

The relay Ri I is intermittently energized by a circuit closed from (-i-l through the keying contact 3, front contact S of relay SQR, windings of relay RI, to The relay SGR is normally energized in a manner later to be explained, so that upon each closure of the keying contacts 8, the relay RII is energized until the keying contacts again open. For convenience in rendering the disclosure denite, the keying contacts are assumed to be closed for a half second every Ythree-and-a-half seconds, but it is to be understood that this timing operation may be varied as desired in accordance with the checking characteristics of the system employed, it being sumcient for an understanding of the present invencontact I0 of relay REI is closed. the checktone Ienergize the relay RI I.

generator is effective to cause the proper control of the transmitter so that its carrier frequency is modulated with a predetermined tone. In other words, a modulated carrier is radiated from the antenna A for the time during which the relay RII is picked up, which occurs intermittently under normal conditions until a voice message is to be transmitted.

In the event that the operator at the central station desires to transmit a voice message to some outlying station, he merely actuates the self-restoring button PB causing the energizetion of the relay R2I. The opening of back contact I0 removes the check-tone generator from control of the transmitter and connects the microphone M. At the same time, iront contact II of relay RZI is closed which supplies energy through front contact 9 of relay SQR. to steadily This renders the radio transmitter active to transmit a carrier which will be modulated in accordance with the voice operation of the microphone M in the usual Way. The energization of indicator lamp I5 through contact I6 of relay RII indicates that the transmitter has been rendered active. When the operator has iinished his message, he releases the button PB which is self-restoring so that the relay R2I is deenergized followed by the deenergization of the relay RII, and relay RII is again dependent upon the operation of the keying contacts.

It is assumed that the receiver at the distant station is equipped in a manner such as disclosed in the prior Patent No. 2,280,420, granted April 21, 1942, to V. C. Chappell, so that upon the cessation of the intermittent reception of the checktone signal, a suitable indication will be displayed and the loudspeaker rendered active. Thus, whenever the check-tone signal fails to be received, the operator at the distant station expects to hear a voice message. If there is no voice message received when the signal controlled by the check tone indicates the absence of a check tone, then the operator is advised that there is some fault in the communication system. Such an organization provides a check' upon the entire system of communication between the central station and the distant station. In other Words, any failure in the system, such as a failure in the microphone at the central station, a failure in the loudspeaker at the receiving station, or any failure at some intermediate point in the system, is immediately brought to the attention of the operator at the distant station.

In the event that the central station receiver is receiving a carrier from the distant eld station, this carrier acts through apparatus. later to be described, to cause the deenergization of the squelch relay SQR. This opens front contact 9 rendering the keyingcontacts 8 ineective, and also closes back contact I2 which renders the amplifying apparatus active so that the loudspeaker LS gives forthA any voice message imposed upon the carrier being received. So long as' this vcarrier continues to be received, the contact 9A is maintained open so that the keying contacts 8 cannot cause the energization of the relay RII and thereby interrupt the operation ofthe receiving apparatus. Also, the open condition of front contact 9 prevents the closure of front contact II (as the result of manual actuation of the foot switch or push button PB) from ei'ecting the interruption of the voice message then being received by a radio receiver. In this Way, the radio transmitting and receiving apparatus is in- Iaffaccio ing stages of the receiver' so as to control the amplification of the carrier signal.

Thus, when the receiver ceases to receive a radio frequency carrier of an amplitude above that which is necessary to .provide the desired operation of the receiver, the AVC circuit becomes automatically effective to render the receiver very sensitive. When this occurs. the receiver would act to reproduce the electrical noise surrounding its location, if it were not for the squelch control. But under such circumstance, the gaseous tube GT becomes conductive because of the absence of a negative potential on its control grid, and the squelch relay SQR is immediately picked up. In other Words, the value of the negative bias required on the control grid of the gaseous tube GT to render it non-conductive is higher than the negative bias produced by the noise level of the particular location of the receiver even with the receiver in its most sensitive condition and with substantially no negative bias on the AVC circuit.

In this way it will be seen that the squelch relay SQR is controlled to be picked up whenever the receiver fails to receive a carrier signal above a predetermined value and then acts to render ineffective the power amplifier stage so that the loudspeaker is mute. This carrier controlled relay SQR also acts to render the associated radio transmitter ineffective to transmit the checking tone signals while the receiver is receiving a radio carrier above the selected amplitude. In this way the squelch relay SQR serves its dual purposes as explained above in connection with Fig. 1.

With reference to Fig. 3 of the accompanying drawings, it Will be seen that a vportion of the radio receiver of Fig. l has been shown in block form while the remaining portion of the receiver has'been shown somewhat in detail. The antenna A and the ground connection G are of course connected to the input of the receiver in a conventional manner, with the antenna selection being made by the contact 6 of the relay RI I and the plate potential being selected through back contact 'I of relay RII as described in connection with Fig. 1. vThe input to the radio receiver is assumed to be applied to a radio frequency amplifier the output of which operates through a first detector or mixer stage to supply an intermediate or beat frequency which passes through an intermediate frequency amplifier to the input of the second detector stage of the receiver. This connection between the intermediate frequency amplifier andthe second detector is eifected inductively in a convention-al manner, the second detector stage being shown as the left-hand portion of a twin-diode tube TD. This left-hand portion of the twin-diode TD is used for detection purposes to provide the input to the audio ampliler and also to produce a negative direct current potential dependent upon the reception of a carrier for controlling .what may be conveniently termed the sq'uelch control portion of the receiver. The right-hand portion of the twin-diode TD is used for providing the automatic volume control with a potential dependent upon the strength of the received carrier signal.

The radio receiver is normally inactive insofar as the reception of a carrier signal is concerned and, for reasons later explained, this results in the normally deenergized condition of the squelch relay SQRI. The open front contact I21 of this relay SQR1 disconnects the cathode of the second audio amplier tube 2-AF from ground which in effect supplies a high negative bias on the control grid of this tube Z-AF, so that this tube is non-conductive, i. e., there is no current flow through its plate-cathode circuit including the primary of a coupling transformer for the loudspeaker LS. v

The first audio amplier stage includes a tube I--AF of the triode 'type while the second stage of audio amplification includes the tube 2--AF of the pentode type.

In connection with the squelch control, two direct current amplifying tubes I-SQ and Z-SQ of the triode type are employed to provide control of the energy supplied to the squelch relay SQR.1 depending upon the reception of a carrier of a substantial value above the average radio noise field intensity of the particular location.

Upon the reception of a carrier signal, it is of course amplified in the radio frequency stage and acted upon in the mixer stage to produce a beat or intermediate frequency in the usual way well known tov those acquainted with superheterodyne radio receivers. When the output of the intermediate amplifier is applied to the lefthand portion of the twin-diode TD, it is rectified to produce a pulsating direct current potential across the condenser C2. A resistor RI in series with the condenser C3 and a resistor R2 in multiple therewith provide what may be termed a radio frequency filtering circuit so that the half cycles of the intermediate or beat frequency will be ltered and the resulting potential at the connection I3 is varied in accordance with the audio frequency modulations imposed upon the carrier. A connection from terminal I3 is made through a radio frequency choke RFC, a condenser C4 and a resistor R3 to ground. The potential across resistor R3 in this circuit is the audio frequency component of the potential appearing at terminal I3 because the condenser CII acts to readily pass such audio frequency component but blocksthe ow ofany direct current component. The resistor R3 is of the variable type and acts as a manual volume control for the audio amplification, so that the desired proportion of the audio frequency potential appearing across the variable resistor R3 is fed through the first stage of audio frequency amplification to the power amplifier in the usual way so that the loudspeaker LS reproduces the audio frequency or voice modulations of the carrier. It may be noted here that the circuits for the heater elements of the cathodes have not'been shown, it being assumed that they are continuously energized so that the tubes are ready for immediate operation.

The rst audio stage is supplied with the usual plate potential from (B+) through a load resistor, and the bias on the grid of tube I--AF is produced by the resistor and condenser arrangement of the associated cathode. A coupling condensor C5 is provided between the first audio stage and the second audio stage which is connected to the loudspeaker LS in the usual Way.

It is to be understood that any suitable audio amplifier organization may be employed, the particular thing to be noted is that normally the cathode of the tube 2-AF is disconnected from ground so that this tube becomes non-conductive and there is no current flow through the platecathode circuit. In this connection it should be noted that the (B+) supplied to the plate of this tube Z-AF could be selected through a front contact ofthe relay SQR1 instead of the ground connection of the cathode, and still'be vWithin the scope of the present invention. `The cathode ground connection is preferably selected, since there is a lower potential to be broken by the contact |21. In other Words, the second audio amplifier stage `is `normally rendered inactive With the squelch relay SQR1 deenergized.

In addition to the audio frequency component of the potential appearing at yterminal I3, there is a -direct Vcurrent component which varies in accordance with the vstrength of `the radio -frelquency carrier signal received, and this direct current component is fed through the resistor R6 'to ground. This variable resistor R5 provides that any desired proportion of such direct -current potential may be supplied through the filtering resistor 35 to the grid of the tube `l-$.i .2. :A condenser C8 provides a by-pass to ground for any audio or radio frequency which vhas not previously been filtered out. This is so that the `tube l-SQ is controlled in accordance with 'the direct current component ofthe negativ-e potential lappearing at the terminal -I3 which vvar-ries in accordance with the strength of the carrier received.

Normally, when vthere is Vno carrier bein-g received, there is no negative rpotential applied to 'the control grid of the tube Al--SGL 4so that the .plate-cathode circuit of this tube V-is normally 'conductive and a 'current flows from (-B+), through the resistors 36 and r3l and the plate- `cathode circuit of the tube l-SQ toground. The current which flows `in Athis circuit causes -a -po- :tentialdropacross the resistors '36 and'fl in such xa Way asto cause a Anegative -biasto be iin-pressed upon 'the control lgrid of the rectifier 2tube .'l-SQ so that its plate-'cathode circuit is normally non- 'conduc'tiva When a carrier is received and a negative potential is applied 'to `the control grid of the 'tube l-SQ, 'and this tube ceases to be conductive, there is no potential drop across atheresistor '31 and the negative bias on Vthe control grid of 'the rectifier tube j2--"SQ, is reduced-to #tl-1e point Where the tube/allowsa Vplate-eatl-iode lcurrent as'supplie'd from the secondary-ef theftransformer 38. lThis,currentiiovvs through Lthe relay SQRl 'and causes its Vcontacts tobe picked lup to render vthe second 4stage 'of the -au'dio amplifier 'to rbe active -for amplifying the 'modulations lof the lreceived carrier. It lshould "be notedfin this `connection l'that a `condenser C9 '-is Yused *in A'connection with the Vr'e'lavSQR-l tofsm-oothout thelpulsations of current in the relay circuit and :provide the better operation of -the relay current vSQP?.

It should be noted that a -resistor-`is-con nected between the-resistors '35 rand-3l lto ground so that a `normal 4bias is `placed v`on the `Ycontrol gridof Athe tube 2-SQ1Whenit is 'acting-'toerectify the Jalternating `current 4applied to the `relay ldircuit. YIt 'is also to-be understood that the various values of the resistors and characteristics of lthe tubes-'should be selected in accordance "Withf'lthe best engineering practices "for the lparticular `embodiment -o'f the invention, the Srillustration being 'shown only `for the purpose o'f 'illustrating the principles of such an organization. Y'Since the characteristics vof Athe tubes .and .their associated vcontrol circuits s'houldbe Aproperly -rdesigned to give 1a `quick response of 'the .relay -SQRl., -this :relay will .readily :act as a-squelch-xcontrol of the amplifier so that upon the cessation of .a `freceived carrier, rthe amplifier-:is immediately rendered :inactive 'andathe JoudspeakerIiS .is ,muted ,It is also 'toy be understoodfthat ,this rrel-ayaSQRl has a contact 1:91 ywhich .operates *.to inter-relate .disclosed in Fig. 4`1.

With a radio receiver thus organized, the receiver ceases to act on the Lloudspeaker until a `radio frequency carrier signal is received of a substantial intensity above the radio noise field intensity of the location of Athe receiver, and this takes Aplaceeven though the receiver is constructed to have an automatic volume con-trol to -make the receiver Isensitive to the -Weakes-t permitted signal.

Although the present invention 'has been shown as associa-ted with ya receiver Organization assumed to be of the superhete-rodyne type for `the reception of Aax-np'l-itude modulated carriers, it is lto be "understood that the principles of the presentinvention may'likew-ise be applied to receivers for frequency modulated carriers. In such an organization, it is `of course Aunderstood that there is no automatic volume eontrolsuoh as vprovided. -hy the right-hand lporti-on ofthe twin-diode but that the volume of such a frequency mod-u- Vlated lreceiver is controlled by reason of what is conventionally termed a limiter stage. I'But even though a receiverfora frequency -rnodulated carrier is employed, the principles of the present invention may still be applied thereto and provide advantageous features. rIlle-is is -because a receiver for a frequency modulated carrier becomes extremely sensitive to the radio Lnoise level as soon as a carrier 4ceases to 3be received and the limiter stage `ceases to be overloaded -by the rey -oeption'of-suoncarrier. For-this reason, a circ-uit organization for .-mut-ng-:the amplifier vand speak- 4er .is especially desirable, and the san-1e functions :of interlock with the ltr-ansmitter vare also ldesirable ianduseful :for the same reasons Ypreviously explained. Therefore, `it Eis :to be understood ithat the present :inventionfis Vcontemplated as being fusefuliin :connection fwithireceivers `for, either amplitudehr frequency imodulatedcarriers.

.In brief, ait )may :be stated thateone-of the principle -characteristics .of :the .present invention kis the .provision Y.of relay means vcontrolled -in accordance with the neceptionof a radio Afreouiency carrier signal :of a :strength kor field intensity i substantially above .the `radio noise ylevel ofthe particular location, fand such relay lmeans acting 4to effect the functions and relationships set forth in detail .above In both forms l.of the invention,

such melay uneans :is controlled aby a direct current voltage developed :at fthe input .terminals of afdiodedetector :in .proportion to the intensity of the received Acarrier :independently .of its modulationicharactenstics.

The :use of such .ia .relay for rendering the am- ;plier inactive "when there ds no carrier .signal being received has the advantage that `:the relay .acts lto ,mute ithe @amplifier .and .loudspeaker abruptly. Thegusuallsquelch circuit with its .tube control Jacts .-gradually and there is `a certain amount yof ,distortion `alluring :this gradual .cut- ,ting-off ,top eration. "Elhus, `the present invention avoids :the 4distortion ,of :this Vthreshold condition. Also, ,the :relayhas the further advantage of saving powerzbymendering theflnaliaudio stage orpovverzamplier completenI `inactive.

Although :the organization ishoyvn gin Fig. ,1 i1- -lustrates hoW:a--1posi'tive interlock is provided to prevent :the :central ,oCe operator `from interrupting 1a message ,ibeing received, vit `should be understood that :insome 'applications of :radio rommunication-fit may"benesirablethat-the cen- .tral office operator :shall abe :permitted '110 -inter- .runt'theg-reeeptionnf la vrlflessaee at any :time for the purpose of transmitting important instructions. This may be accomplished by connecting from front contact Il to the left of contact 9 so that relay RI I can repeat the relay R21 irrespective of the condition of relay SQR. It should be understood that this change may be made Within the scope of the present invention without in any Way aiecting the other features and characteristics of the invention.

The feature herein disclosed relating to silencing or squelching the audio portion of a radio receiver when no carrier signal is being received is claimed in the divisional application Ser. No. 742,010, led April 17, 1947.

Having described a radio communication system embodied into two diierent forms as constituting the present invention, it is to be understood that these forms are selected to facilitate in the disclosure of the invention rather than to limit the number of forms which it may assume; and, it is to be further understood that various modications. adaptations and alterations may be applied to the specie forms shown to meet the requirements of practice, Without in any manner departing from the spirit or scope of the present invention.

What We claim is:

1. In a radio transmitting and receiving system wherein the transmitter intermittently transmits a carrier signal for checking communication With a distant station and wherein a transmitter at the distant station at times transmits a modulated carrier signal to said receiver which includes an audio amplier for supplying the input to a loud speaker, a triode electronic tube in addition to said audio amplier, a relay connected in the plate-cathode circuit of said tube and controlled by the current flow therein to govern the output of said audio amplifier, means for governing the negative bias on the control grid of said tube in accordance with the intensity of a received carrier signal to render said relay effective to mute said loud speaker when the received carrier ceases, and means acting when said relay is controlled by the reception of a carrier signal to allow the operation i' said loud speaker and t0 also render the associated transmitting means inelective to transmit said intermittent carrier signals.

2. In a radio transmitting and receiving organization, a transmitter acting automatically t0 intermittently transmit carrier signals for checking communication with a distant station, a multiple stage receiver associated with said transmitter, a grid controlled thyratron tube, relay means associated with said tube, a plate-cathode circuit for said tube including said relay7 means and a source of alternating current energy in series, a control circuit for the grid of said tube supplied with a negative bias by said receiver when a carrier signal is being received by the receiver to thereby render said thyratron tube nonconductive, and circuit means controlled by said relay means when deenergized for preventing the automatic operation of said transmitter to intermittently transmit said carrier signals.

3. In a radio transmitting and receiving organization, a transmitter acting automatically to intermittently transmit carrier signals for checking communication with a distant station, amultiple stage receiver associated with said transmitter and including a diode detector, a grid controllecl thyratron tube, relay means associated with said tube, a plate-cathode circuit for said tube including said relay means and a source of alternating current energy in series to thereby normally energize said relay means, a circuit for the control grid of said tube supplied with a negative bias in accordance with the intensity of a signal supplied to the input of said diode detector whenever a carrier signal is being received by the receiver to thereby render said thyratron tube non-conductive and cause the deenergization of said relay, and circuit means controlled by said relay means when deenergized for preventing said automatic operation of said transmitter to intermittently transmit said checking carrier signals.

4. In a radio transmitting and receiving organization, a transmitter acting automatically to intermittently transmit a carrier signal for at intervals checking communication with a distant station, a. multiple stage receiver associated with said transmitter and including an audio amplifier for supplying the input of a loudspeaker, a grid controlled thyratron tube associated with said receiver, a direct current relay, a platecathode circuit for said tube including said relay and a source of alternating current in series to thereby normally energize said relay, a condenser for shunting said relay, a circuit for the control grid of said tube supplied with a negative bias by said receiver when a carrier signal is being received to thereby render said thyratron tube nonconductive and cause the deenergization of said relay, `and circuit means controlled by said relay when energized for permitting the automatic operation of said transmitter to intermittently t1 ansmit checking carrier signals but rendering said audio ampliier inactive to thereby mute said loud speaker, said means acting to prevent said automatic operation of said transmitter when said relay is deenergized and at the same time acting to allow said audio amplier to be active for reproducing the modulations of a received carrier signal.

5. In a radio transmitting and receiving organization, a transmitter acting to automatically transmit a carrier signal at recurring intervals for checking communication with a distant station, a multiple stage receiver associated with said transmitter and including a diode detector and an audio amplifier for supplying the input to a loud speaker, a multiple stage squelch system including a rst and a second triode tube, circuit means for applying a direct current negative potential to the control grid of the rst triode when a carrier signal is received by the receiver and supplies an input to said diode detector to thereby cause said first triode to be non-conductive when a carrier signal is being received, circuit means controlled by the normal plate current of said rst triode while no carrier signal is being received for normally negatively biasing the control grid of said second triode beyond cut-off but changing the negative bias on said control grid of the second triode to a normal operating value when the plate-cathode circuit of said first triode becomes non-conductive, a plate-cathode circuit for said second triode including a relay and a source of alternating current, and circuit means acting to render said audio amplier inactive when said relay is deenergized and also acting to stop said automatic operation of said transmitter when said relay .is energized.

6. In a radio transmitting and receiving organization adapted to operate on the same radio frequency channel, a transmitter acting to automatically transmit a carrier lsignal at recurring intervals for checking communication with a distant station, a multiple stage receiver associated with said transmitter and including a diode detector and an audio ampliiier for supplying the input to a loudspeaker, a multiple stage carrier responsive system including a first and a second triode tube, circuit means for applying a direct current negative potential to the control grid of the first triode when a carrier signal is received by the receiver and supplies an input to said diode detector to thereby cause said iirst triode to be non-conductive when a carrier signal is being received, circuit means controlled by the normal plate current of said rst triode while no carrier signal is being received for normally negatively biasing the control grid of said vtriode beyond cut-01T but changing the negative bias on said control grid of the second triode to a normal operating value when the plate-cathode circuit of said rst triode becomes non-conductive, a direct current relay, a plate-cathode circuit for said second triode including said relay and a source of alternating current, and circuit means acting to stop said automatic operation of said transmitter when said relay is energized, whereby the reception of a carrier signal between intervals of check-tone modulated carrier transmission renders the automatic operation of said transmitter ineiective to thereby allow continued reception of said carrier signal without interference.

7. In a radio transmitting and receiving organization, a transmitter normally acting to intermittently transmit a carrier signal at recurring intervals for checking communication with a distant station, manually operable means for rendering said transmitter eiective to transmit voice messages and at the same time preventing the transmission of said checking carrier signals, a multiple stage radio receiver associated with said transmitter and adapted to receive carrier current signals over the same carrier frequency channel as employed by said transmitter, said receiver being normally effective to receive signals but rendered ineiiective whenever said transmitter is either manually or automatically set into operation, a grid controlled gas discharge tube associated with said receiver, a direct current relay. a plate-cathode circuit for said gas tube including said relay and a source of alternating current, a circuit for the control grid of said tube normally having substantially no negative bias but being supplied by a negative bias when a carrier signal is received by said receiver to thereby render the plate-cathode circuit nonconductive. and circuit means controlled by said relay when deenergized for preventing either the automatic or the manual operation of said transmitter.

8. In a radio transmitting and receiving system. a radio transmitter normally acting to intermittently transmit a carrier signal at recurring intervals for checking communication with a distant station, manually operable means for at times rendering said transmitter effective to 14 transmit voice modulated carrier signals and at the same time preventing the transmission of said checking carrier signals, a multiple stage radio receiver associated with said transmitter and adapted to receive modulated carrier current signals over the same carrier frequency channel as employed by said transmitter, said receiver being normally eiective to receive signals but rendered ineffective whenever said transmitter is either manually or automatically set into operation, a grid controlled triode tube associated with said receiver, a direct current relay, a platecathode circuit for said tube including said relay and a source of alternating current in series, a circuit for the control grid of said tube supplied with a negative bias in accordance with the reception of a carrier signal by said radio receiver to thereby control said relay in accordance with the reception of signals by said receiver, and circuit means controlled by said relay for preventing either the automatic or the manual operation of said radio transmitter during the reception of a signal by said radio receiver.

9. In a radio transmitting and receiving system, a radio transmitter normally acting to intermittently transmit a carrier signal at recurring intervals for checking communication with a distant station, manually operable means for at times rendering said transmitter effective to transmit voice modulated carrier signals and at the same time preventing the transmission of said checking signals, a multiple stage radio receiver associated with said transmitter and adapted to receive modulated carrier current signals over the same carrier frequency channel as employed by said transmitter, said receiver being normally eiiective to receive signals but rendered ineffective whenever said transmitter is either manually or automatically set into operation, a grid controlled triode tube associated with said receiver, a direct current relay, a plate-cathode circuit for said tube including said relay and a source of alternating current in series, a circuit for the control grid of said tube normally having a negative bias below the cut-off of said tube supplied by said radio receiver to thereby deenergize said relay, but said circuit having such negative bias removed during the reception of a carrier signal by said receiver to thereby cause the energization cf said relay, and circuit means controlled by said relay when energized for preventing either the automatic or the manual operation of said radio transmitter.

JAMES W. KNAPP. JOHN C. OBRIEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,114,036 Smith et al. Apr. 12, 1938 2,115,813 Jarvis May 3, 1938 

